Will be wearing Uniqlo down-filled jacket and Heattech shirt, how many more layers of clothes will be needed to keep warm outside?
I have been to Hokkaido in January and February and experienced harsh coldness.
For tops, get a Heattech fleece turtleneck shirt. It helped me a lot to warm my neck. Adding one more layer of knit or sweater under Uniqlo down jacket is a good idea. For bottoms, wear Heattech tights (knit tights are better) under your pants. Plus one more socks if you are going to be outside for a long time, especially at night. Don't forget hat, scarfs, and gloves, too.
It is better to prepare as thick as you can think of. Getting "Hokkairo" and sticking on your shirt is a good idea, too.
Ski suit and hat, gloves, boots, cold weather wear, always wearing. Ideally, if face is almost not externals.